1. Field of the Invention
The present invention relates to photoelectric instruments used to monitor a field of view and to detect non-uniform intrusions into the field of view.
2. Description of the Prior Art
There are numerous applications for photoelectric devices which monitor a field of view and which provide signals indicative of movement within the field of view. For example, video cameras are used to provide unmanned surveillance of areas for security purposes. Photosensors are used to detect the approach of containers in cargo loading and unloading systems and to detect the movement of material into position to be processed, such as, for example, in a wood cutting or milling operation. Photoelectric sensors are also utilized to control the operation of elevator doors, burglar alarms, conveyor belts and in other applications where an automatic signal is required in response to movement within a field of view.
Heretofore, very sophisticated instruments have been required to view all but the shortest fields of view. Simple photoelectric cells have heretofore been useful for area surveillance only at a range of a few feet, and only where uniform ambient light conditions exist. While video cameras are useful in conducting area surveillance, they must be manually monitored either directly, or by means of video tape, in order to detect the occurrence of events of interest. The cost of video cameras is such that often times the expense of a video camera system is not warranted by the task to be performed. Less sophisticated area surveillance systems, on the other hand, require a highly collimated light source at a short distance in order to be able to discern movement or protrusion of an obstacle into a field of view.